The long-term objectives of this research are to employ a cognitive neuropsychological model to conceptualize the effects of HIV and drug abuse on the brain and to relate these effects to critical behaviors in HIV-seropositive persons such as high risk sexual and injection practices. Evidence indicates that drug abuse and HIV infection cause dysfunction in overlapping frontal-subcortical brain regions and that careful study of specific cognitive functions mediated by different frontal-subcortical circuits will permit characterization of HIV-related cognitive impairment and its manifestation in persons who abuse drugs. This study will begin to evaluate the question of whether a history of drug abuse in the context of HIV infection will increase vulnerability to HIV-related cognitive deficits, a question with potential diagnostic and therapeutic implications. The specific aims of this investigation are to compare the performance of HIV-seropositive (HIV+) and HIV- seronegative (HIV-) with a current or past history of drug abuse on measures of decision-making, working memory, and response inhibition, "executive" functions dependent on the integrity of prefrontal-subcortical pathways and potential mechanisms of HIV- related cognitive dysfunction; to evaluate the relationship between parameters of drug abuse severity to deficits in executive function in HIV+ and HIV - drug users; to evaluate the presence and severity of executive deficits in HIV+ drug users at different stages of HIV disease; and to evaluate the relationships between executive functions and sexual and drug use-related risk behavior. The study will evaluate cognitive function in HIV+ and HIV- drug users employing state of the art computerized measure developed to model specific impairments associated with dysfunction of frontal-subcortical circuits. Data from the proposed study will advance knowledge of cognitive aspects of HIV-1 infection; the practical implications of how cognitive impairment associated with HIV influences behavior, and recent attempts to bridge the gap on how specific cognitive impairment can influence risk behavior